1. Field of the Invention
The invention relates to an optical device, and more particularly to an optical device comprised of an optical unit or a modularized optical unit which optical unit includes a plurality of optical circuits such as a Mach-Zehnder interferometer.
2. Description of the Related Art
It is necessary in a light transmission system to densify an interval between channels or increase a transmission rate in order to increase transmission capacity. A light communication system satisfying such requirement would have to include various optical devices such as a device for merging wavelengths of optical signals into one another or separating a wavelength of an optical signal from merged wavelengths, a device for compensating for dispersion, or a gain equalizer used for an optical fiber amplifier.
Some devices among those optical devices have an optical circuit which is designed to have multi-stages by virtue of planar lightwave circuit (PLC) technology. As an example of such devices, hereinlater is explained a Mach-Zehnder interferometer type optical circuit.
FIG. 1 illustrates a structure of a Mach-Zehnder interferometer type optical circuit fabricated on a substrate.
The illustrated Mach-Zehnder interferometer type optical circuit 11 is an asymmetric Mach-Zehnder interferometer type optical circuit. The asymmetric Mach-Zehnder interferometer type optical circuit 11 fabricated on a substrate 12 is comprised of a shorter arm 13, a longer arm 14 arranged above the shorter arm 13, a first directional coupler 151 optically connected to inputs of the arms 13 and 14, and a second directional coupler 152 optically connected to outputs of the arms 13 and 14.
A multi-mode interference (MMI) coupler may be used in place of the directional couplers 151 and 152.
The asymmetric Mach-Zehnder interferometer type optical circuit 11 is designed to receive an optic signal through one or all of first and second optic waveguides 16 and 17, and output a desired optic signal. It is assumed hereinafter that the asymmetric Mach-Zehnder interferometer type optical circuit 11 is used as a device for merging wavelengths of optic signals into one wavelength or a device for separating an optic wavelength from merged wavelengths.
Herein, it is assumed that the asymmetric Mach-Zehnder interferometer type optical circuit 11 receives a forty-channel optic signal 18 through the second optic waveguide 17.
The first directional coupler 151 divides the optic signal 18 into two forty-channel signals 21 and 22, and then, transmits the thus divided signals 21 and 22 to the longer arm 14 and the shorter arm 13, respectively. The forty-channel signals 21 and 22 enters the second directional coupler 152 through the longer arm 14 and the shorter arm 13, and then, interfere with each other in the second directional coupler 152 by a phase difference equivalent to a difference in length between the longer arm 14 and the shorter arm 13.
As a result, the first optic waveguide 16 outputs an optic signal 23 having K-th channels (wavelengths) including second to fortieth channels wherein K is an even integer, and the second optic waveguide 17 outputs an optic signal 24 having M-th channels (wavelengths) including first to thirty-ninth channels wherein M is an odd integer.
Though the asymmetric Mach-Zehnder interferometer type optical circuit 11 illustrated in FIG. 1 is of a single stage, a Mach-Zehnder interferometer type optical circuit may be designed to have a plurality of stages optically connected to one another. A Mach-Zehnder interferometer type optical circuit having a plurality of stages could have enhanced characteristics as an optical filter by varying parameters, or could have complex functions.
There has been suggested an optic finite impulse response (FIR) filter comprised of a plurality of Mach-Zehnder interferometers optically connected to one another in multi-stages, and a plurality of phase-shifters through which the Mach-Zehnder interferometers are optically connected to one another.
FIG. 2 illustrates an optical device including a plurality of Mach-Zehnder interferometer type optical circuits arranged horizontally in a line.
The illustrated optical device is comprised of N pairs of arms, and first to (N+1)-th directional couplers 151 to 15(N+1) through each of which adjacent pair of arms is optically connected to each other. Each pair of arms is comprised of a shorter arm 131 to 13N and a longer arm 141 to 14N.
It is not always necessary for the shorter arms 131 to 13N to be equal to one another in N pairs, and similarly, it is not always necessary for the longer arms 141 to 14N to be equal to one another in N pairs, because a difference in length between the shorter and longer arms in each of N pairs defines a phase difference.
The first to (N+1)-th directional couplers 151 to 15(N+1) are horizontally directed, that is, in a direction in which a light goes on.
The optical device illustrated in FIG. 2, including the Mach-Zehnder interferometer type optical circuits arranged horizontally in a line, is accompanied with a problem that the optical device unavoidably is horizontally too long with the result in an increase in a size of the optical device. Accordingly, a yield at which optical devices can be fabricated from a substrate would be reduced.
FIG. 3 illustrates an optical device including a plurality of optical circuits arranged for increasing a yield.
In the illustrated optical device, the shorter arms 131 to 13N and the longer arms 141 to 14N are designed to be bent in the same direction in each of N pairs, but the shorter and longer arms in adjacent pairs are designed to be bent in opposite directions. The first to (N+1)-th directional couplers 151 to 15(N+1) through which adjacent pairs of the shorter and longer arms are optically connected to each other are vertically directed, that is, in a direction perpendicular to a direction in which the N pairs of the shorter and longer arms are optically connected to one another.
As is understood soon in view of FIG. 3, the optical device has a zigzag shape, and is shortened in a horizontal length in equivalence to an increase in a vertical length in comparison with the optical device illustrated in FIG. 2.
However, even in the optical device illustrated in FIG. 3, including a plurality of the Mach-Zehnder interferometer type optical circuits arranged in a zigzag configuration, the optical device would unavoidably have an increased horizontal length if the optical device had an increased number of the Mach-Zehnder interferometer type optical circuits. This results in reduction in a yield at which optical devices are diced out of a substrate, similarly to the optical device illustrated in FIG. 2. The reduction in such a yield results in an increase in fabrication costs of an optical device.
In addition, since a substrate has unevenness in a profile of an index of refraction, there is unevenness in characteristics in different areas of the substrate. Hence, the optical device is accompanied further with a problem that it is more difficult to ensure constant quality in optical devices, if those optical devices were to be diced out of a substrate having a larger size.
Though the problems in a Mach-Zehnder interferometer type optical circuit as an example have been explained above, an optical device comprised of a single or a plurality of circuit(s) formed on a common substrate and optically connected to one another would be accompanied with the same problems as mentioned above.
Japanese Unexamined Utility Model Publication No. 58-59205 (U) has suggested a directional coupler having a coupling area sandwiched between two micro-strip paths which coupling area has a length of ¼ wavelength. The two micro-strip paths are formed on a dielectric substrate in a spiral. Coupling areas are formed at opposite sides of at least a part of the micro-strip paths.
Japanese Unexamined Patent Publication No. 1-191803 (A) has suggested an optic gyro including a glass substrate or an optic monocrystal substrate, and an optic waveguide formed on the substrate in a spiral. The optic waveguide has leading and trailing edges at an outer edge of the substrate.
Japanese Unexamined Patent Publication No. 5-181028 (A) has suggested an optic ring oscillator including a ring-shaped optic path, optic couplers, and at least one optic input and output path optically connected to the ring-shaped optic path through the optic couplers. At least one of the optic couplers can vary an intensity of input and output optic signals.
Japanese Unexamined Patent Publication No. 8-279646 (A) has suggested a pulse-light source including a light waveguide circuit having two inputs and two outputs and comprised of two light waveguides having portions arranged close to each other, and a plurality of directional couplers arranged in the portions, a phase-modulator optically connected to one of the two inputs, and a laser optically connected to the phase-modulator. The light waveguide circuit includes a plurality of first waveguide areas in each of which the two light waveguides are equal in length to each other and a second waveguide area in which the two light waveguides are not equal in length to each other. The second waveguide area is optically connected to at least one first waveguide area both at a side close to the laser and at a side remoter from the laser.
Japanese Unexamined Patent Publication No. 2001-109022 (A) has suggested an add-drop filter including a plurality of Mach-Zehnder interferometers having two inputs and two outputs and including two directional couplers or 2×2 MMI couplers through which two light waveguides formed close to each other on a substrate are optically connected to each other. Each of the Mach-Zehnder interferometers is designed to include an optically inductive grating or a heater in at least one of arms of the light waveguides sandwiched between the directional couplers or the 2×2 MMI couplers.